1. Field of the Invention
The present invention relates to a digital differential signal transmitter for operation under a low supply voltage, and more particularly, to a digital differential signal transmitter circuit for a low supply voltage wherein a phase correction circuit for correcting digital signals transmitted through two signal paths in such a way as to have a phase relationship of differential signals and duty cycle correction circuits for correcting the digital signals in such a way as to maintain signal integrity in spite of changes in process, supply voltage and temperature are installed on the two signal paths so that the distortion of digital differential signals is compensated for, wherein power consumption at a final output section of the transmitter circuit is reduced, and wherein impedances of the transmitter circuit and transmission lines are matched so that the transmitter circuit can operate insensitively with respect to operation circumstances.
2. Description of the Related Art
In general, differential signals mean that signals having the same information have a phase difference of 180°. These differential signals should have a phase difference of 180° while being transmitted through a circuit and should be maintained in such a way as to conserve the same information between the two signals.
In the case of an analog circuit, since the transmission of differential signals is implemented through a circuit having the form of a differential pair, the phase difference between two signals and the information of the signals can be easily maintained.
However, in the case of a digital circuit, it is difficult to ensure the signal integrity of the differential signals, except a CML (current mode logic) circuit which is configured to toggle with a preset amplitude or a preset swing range on the basis of a predetermined DC level or an average level determined by a certain reference. Nevertheless, in the case of the CML circuit, due to the structural characteristics thereof, limitations exist in decreasing a supply voltage so as to reduce power consumption, and the circuit itself consumes a substantial amount of power, whereby it is difficult to use the CML circuit unless a circuit dedicated for high speed operation is needed. Therefore, problems are caused in that the CML circuit is difficult to be used in a wired digital differential signal transmitter for operation under a lower supply voltage.
Accordingly, in a conventional wired digital differential signal transmitter, a CMOS (complementary metal-oxide semiconductor) circuit is generally used. That is to say, in the case of the CMOS circuit, since all MOS elements in the circuit operate as switches and there is no static power consumption, the CMOS circuit is appropriate for the design of a circuit for operation under a low supply voltage and is therefore used in the conventional wired digital differential signal transmitter.
FIG. 1 is a view illustrating the configuration of a conventional CMOS type wired digital differential signal transmitter.
Referring to FIG. 1, a conventional CMOS type wired digital differential signal transmitter is configured to transmit input differential signals Din and Dinb using CMOS inverters 20 so as to operate output driving inverters 10. At this time, in the conventional CMOS type wired digital differential signal transmitter, because there is no interference between two signal paths for transmitting the differential signals, the operational characteristics of the signal path of Din and the signal path of Dinb are likely to vary by a change in process, supply voltage, operation temperature, etc.
Hence, while the input differential signals Din and Dinb have the characteristics of differential signals, since the transmitter cannot maintain the signal integrity, a problem is caused in that output signals Dout and Doutb do not have the characteristics of differential signals.
FIG. 2 is a view illustrating a state in which input differential signals are outputted by being distorted in the conventional transmitter.
Referring to FIG. 2, since the transmitter cannot maintain the signal integrity even under a change in process, supply voltage, operation temperature, etc., the input differential signals are distorted. Therefore, it is to be understood that the transmitter cannot but transmit the differential signals distorted in this way as output differential signals.
Also, as can be readily seen from FIG. 2, if the signals to be transmitted by the transmitter are distorted and lose the characteristics of differential signals, the advantages of differential signals which operate insensitively with respect to noise cannot be rendered, and the probability of signal transmission errors to occur increases.
Recently, in order to solve these problems caused in the conventional wired digital differential signal transmitter, improved type digital differential signal transmitters of which characteristics are ameliorated in terms of signal distortion have been proposed in the art.
FIG. 3 is a view illustrating the configuration of an improved type digital differential signal transmitter which can solve the distortion of signals in the transmitter shown in FIG. 1.
Referring to FIG. 3, in order to solve the problems caused in the conventional digital differential signal transmitter in terms of differential signal integrity, an improved digital differential signal transmitter includes one or more cross-coupled inverters 30 which are arranged between two signal paths for transmitting input differential signals Din and Dinb using CMOS inverters 20 such that the cross-coupled inverters 30 can correct the signals distorted during passage through a plurality of inverters 10 to approach differential signals.
Nonetheless, in the improved type digital differential signal transmitter, the cross-coupled inverters 30 arranged between the two signal paths can cause short-circuit current between a supply voltage (VDD) and a ground (GND) so that power consumption increases. Moreover, since the correction of the input differential signals is influenced by a change in the driving capability of the cross-coupled inverters, a signal correction degree can vary depending upon a change in process, supply voltage, operation temperature, etc.
Further, in the case where the driving capability of the cross-coupled inverters are too great, the cross-coupled inverters can operate in such a way as to disturb the change of the input differential signals Din and Dinb between the two signal paths, whereby the high speed operation of the transmitter is likely to be adversely influenced.
In addition, due to the fact that the conventional transmitters shown in FIGS. 1 and 3 have a final output section which has a push-pull structure of a PMOS and an NMOS, limitations exist in decreasing a supply voltage of the final output section so as to reduce power consumption.
As a consequence, the conventional CMOS digital differential signal transmitters cannot sufficiently compensate for the distortion of differential signals due to a change in the manufacturing process of a chip, supply voltage, operation temperature, etc., whereby the performance of the transmitters can deteriorate. In particular, under a low supply voltage, problems are still caused in that the transmitters sensitively react on the change of the operational circumstances.